Windmills that have conventionally been used as wind power generators include those that are each provided with a nacelle that is rotatably installed on the top portion of a tower and in which electricity generators and so on are disposed, and blades that are rotatably installed on a hub (main shaft portion) provided on the nacelle. In such a windmill, yaw drive units, pitch drive units, or the like are installed as drive units for a windmill that drive one structure to rotate relative to another structure in a movable portion of the windmill. The yaw drive units are configured to drive the nacelle, which is the one structure, to rotate relative to the tower, which is the other structure, thus allowing the nacelle to turn in accordance with the wind direction. The pitch drive units are configured to drive shaft portions of the blades, each of which is the one structure, to rotate relative to the hub on the nacelle side, which is the other structure, thus enabling control of the pitch angles of the blades.
Patent Document 1 discloses a drive unit for a windmill that serves as a yaw drive unit that includes an electric motor and a pinion that is attached to an output shaft of the electric motor and outputs a rotational driving force, and rotationally drives the nacelle by causing the pinion to mesh with a ring gear that is fixed to the top portion of the tower. Patent Document 1 also discloses a braking mechanism for stopping the output shafts of electric motors of two drive units for a windmill by simultaneously driving these electric motors in opposite directions of rotation.